Howdy
My 'Iron Framed' Uberti Henry is only a few years old. When I slugged the barrel it came out as .429. This is the figure I see reported most often for these rifles. Interestingly enough, because I have other 44-40 rifles with .427 barrels, including an Uberti 1873 made in the 1980s, I have compromised at .428 for the diameter I size my 44-40 bullets too. My .429 Henry does just fine with these bullets even though they are nominally .001 undersize for the barrel.
This is probably because even at .001 undersize, the rifling still gets a good grip on the bullet. It may also be because I use dead soft, pure lead bullets, and they may be bumping up slightly in diameter in the bore.
I have never put anything but Black Powder through my Henry. My load is 2.2CC of FFg under a 200 grain Mav-Dutchman Big lube bullet. This comes to about 35 grains when the charge is weighed, being compressed between 1/16" - 1/8" when the bullet is seated. Even though my Henry has a steel frame, I would not hesitate to use this load in a brass framed Henry. They are proofed at Government proof houses with loads that exceed the standard SAAMI spec by about 30%.
P.S. You are correct that Winchester went to an iron frame with the '73 when they first brought out the Model 1873 in 44-40 because the powder capacity had been increased from about 28 grains in the 44 Henry Rimfire round to about 40 grains in the 44-40. However as long as you do not exceed Max SAAMI loads a brass framed Henry will be fine with any loads, BP or Smokeless.
P.P.S. Mike Venturino does comment in his book Shooting Lever Guns of the Old West that he saw a brass framed Henry that had its headspacing ruined by less than a box of 'hot' 44-40 rounds.